Simultaneous measurement of thermal diffusivity and optical absorption coefficient of homogeneous solids using photothermal radiometry

R. Fuente1, E. Apiñaniz1, A. Mendioroz1 and A. Salazar1

1Fisika Aplikatua I Saila, Euskal Herriko Unibertsitatea, Spain

Keywords: photothermal radiometry
property: thermal difussivity, optical absorption
material: -

Modulated photothermal radiometry consists of illuminating the sample by an intensity modulated light beam and detecting the oscillating component of the temperature rise by means of an infrared detector connected to a lock-in amplifier. As the temperature rise depends on the thermal properties of the sample, this technique has been used to measure the thermal diffusivity of a wide variety of materials.

The aim of this work is to test the ability of the modulated photothermal radiometry to retrieve simultaneously and accurately both the thermal diffusivity (D) and the optical absorption coefficient (α) in homogeneus slabs. We first analyze the theory of photothermal radiometry signal generation, including some additional effects such as the multiple reflection of the exciting light beam, the influence of heat losses and the transparency of the sample to infrared wavelengths. Then, we perform measurements on a wide variety of materials: (a) opaque to visible and infrared wavelengths (glassy carbon, metals, stainless steel, SiC, carbon-fiber reinforced composites,...), (b) opaque to visible but semitransparent to infrared (LaMnO3, CoO, Cr2O3, NiO,...), (c) semitransparent to visible but opaque to infrared (neutral density filters), and (d) semitransparent to both visible and infrared (PEEK polymer, coloured filters,...).

The method consists in measuring the amplitude and phase of the signal as a function of the modulation frequency and comparing the results with the theoretical model. However, it is necessary a normalization procedure in order to suppress the instrumental dependence on the frequency. The self-normalization method (the ratio between the rear and the illuminated surface temperatures) provides the highest signal to noise ratio and amplitude and phase contrast.

According to this study, D and a can be obtained simultaneously and accurately provided 0.8 < αL 10 (opaque) only the thermal diffusivity can be obtained. On the other hand, if αL < 0.8 (transparent) neither D nor a can be retrieved.

This work has been supported by the Ministerio de Ciencia e Innovación (MAT2008-01454).

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